1. Field of the Invention
The present invention relates to an electric circuit device having a chip mounted in a face-down orientation on a circuit board. Here, the term xe2x80x9cchipxe2x80x9d refers to any electrical component, such as, but not limited to, components having rectangular casings.
2. Description of the Related Art
FIG. 1 illustrates a related method employed when a chip 2 is mounted on a circuit board 1, such as a printed circuit board or the like. The circuit board is formed using an epoxy resin or the like as a base material. A hybrid integrated circuit can be formed on the circuit board using the so-called xe2x80x9csolder bumpxe2x80x9d technique. More particularly, a chip 2 is mounted in a face-down orientation by bonding the solder bumps 4 of the chip 2 to an electrode 3 formed on the circuit board 1. The electrode 3 of the circuit board 1 is coated with a solder resist film 5 so that the solder resist film 5 prevents a solder from wetting and unnecessarily spreading on the electrode 3 when the solder is applied. After the solder bumps 4 are bonded as described above, the board is cleaned by removing the flux. A sealing resin 6 is discharged to the chip 2 on one side end of the chip, or near one side or two sides of the chip by means of a dispenser 7 or the like. The solder then flows into the gap between the chip 2 and the circuit board 1 based on the capillary phenomena. Then, the resin is heat-cured or photo-cured for making a seal with the resin (see FIG. 2). In FIGS. 1 and 2, just the chip 2 is mounted on the circuit board 1 for ease of illustration. However, other surface mounted components such as chip capacitors, chip resistors, and the like can be mounted adjacent to the chip 2.
In the hybrid integrated circuit as described above, the resin 6 becomes a smooth fillet 6a due to its surface tension. The fillet 6a bulges outward from the chip 2. When the board 1 is heated, or the resin 6 having a low viscosity is used to enhance the flow of the resin 6, the surface tension of the resin 6 decreases. As a result, the fillet 6a can enlarge and spread out, or the resin 6 is formed into a pseudo-circular shape at the location where the resin was initially discharged from the dispenser. This is an undesirable shape for resin sealing. Further, in some cases, variations in the discharge quantity of the resin from the dispenser 7 cause the fillet to become enlarged. When the resin 6 is considerably spread outward from the chip 2, as described above, the resin 6 may cover the electrode surface on which a component is to be mounted. This causes problems because the electrical connection is impaired. Also, the danger of the resin spreading hinders producing a device having high mounting density.
Accordingly, it is one exemplary object of the present invention to solve the above-described problems and to provide an electronic circuit device in which the undesired spread of resin is prevented and on which components can be mounted at a high mounting density.
To achieve the above object, according to a first exemplary aspect of the present invention, there is provided an electronic circuit device having at least one chip mounted in a face-down orientation on a circuit board. An enclosure for preventing a resin from flowing outside the enclosure is provided. The enclosure is larger in spatial extent than the chip provided in a chip mounting area of the circuit board. That is, the perimeter of the enclosure is larger than the perimeter of the chip. The chip is mounted in a face-down orientation in the chip mounting area surrounded by the enclosure, and a sealing resin is introduced into the gap between the chip and the circuit board.
When the sealing resin flows into the space defined by the chip and the enclosure, the resin flows into the gap between the chip and the circuit board. During this process, the sealing resin is preventing from flowing outward from the chip beyond the boundaries of the enclosure. Therefore, even though a resin having a low viscosity may be used, or the discharging quantity of the resin is increased to some degree, the enclosure prevents the resin from spreading out in an undesired manner, so that the resin is prevented from covering an electrode of an adjacent electronic component. Thus, it becomes possible to mount components with a high density and to easily introduce the resin.
When the chip and the enclosure are close to each other, the resin tends to overflow onto the chip and the enclosure. According to a second aspect of the present invention, a resin-casting gate protruding outward from the enclosure is provided. The gate is connected to the enclosure, such that the resin is prevented from overflowing in a direction outward from the chip.
In a typical case, it takes about 10 to 15 seconds for the sealing resin to flow into the gap between the circuit board and the chip. As the size of the chip is increased, the flow time becomes longer. In order to confirm whether the flow of the resin is completed or not, it would be necessary to monitor the four peripheral sides of the chip. This makes it difficult to realize a more efficient process. Particularly, when the distance between the chip and the enclosure provided along the periphery of the chip is short, it is difficult to judge whether the introduction of the resin is completed or not. Thus, according to a third aspect of the present invention, preferably, a groove for checking resin-introduction is provided. When the resin flows into of the groove, the resin introduction should be stopped. In one embodiment, the groove is provided at the farthest position from the gate. In other words, the flow of the resin is slowest at the position opposite to the gate (e.g., at the farthest position from the gate). Accordingly, when the resin flows into of the groove, it shows that the introduction of the resin has been completed. Thus, it can be easily judged whether the charging of the resin is completed or not.
According to a fourth aspect of the present invention, the enclosure is formed with a solder resist film. That is, when the present invention is applied to a hybrid integrated circuit, a surface mounted component is solder-bonded to the circuit board, adjacent to the chip. It is desirable to coat the solder resist film on the periphery of the area where the surface mounted component is solder-bonded so that the solder is prevented from spreading into an undesired or unnecessary place. By utilizing the solder resist film as the enclosure for preventing a resin from flowing out of the enclosure, the process for forming a separate enclosure can be omitted.
It is desirable to provide a predetermined clearance between the enclosure and the chip, so as to form the enclosure with a high dimensional accuracy. In case the enclosure is formed with an ordinary thermosetting resin by a printing method, dimensional variations of about xc2x10.1 to 0.2 mm are generated. When the enclosure is formed with a photosensitive resin material by photolithography according to a fifth aspect of the present invention, the dimensional variation can be reduced to be within xc2x10.05 mm. Thus, the dimensional accuracy of the enclosure can be enhanced.
When a metallic material such as Au or the like is used for the bumps, the height of the bumps is about 20 to 50 xcexcm. The bumps are bonded to the circuit board by a technique such as thermocompression bonding or the like. In the process, the bumps are crushed under pressure, resulting in the formation of a gap with a height of about 10 to 30 xcexcm between the chip and the circuit board. The sealing resin is cast into this gap. The distance between the side of the chip and the wall of the enclosure is narrow, for example, about 200 to 300 xcexcm. Accordingly, if the top of the enclosure is disposed above the top of the chip, the resin is distributed in an unsatisfactory manner, causing a problem that the resin will flow along the top of the chip and the enclosure. For this reason, according to a sixth aspect of the present invention, preferably, the top of the enclosure is lower than the top of the chip.